What is a no-stop limit?

The no-decompression limit (NDL), or no-stop limit, is the maximum time you can spend at a given depth and still ascend directly without a required decompression stop. Deeper means a shorter no-stop limit.

What is a pressure group?

A letter or level that represents how much theoretical nitrogen is still in your tissues. Deeper and longer dives put you in a higher group; time on the surface moves you to a lower one.

What is residual nitrogen time?

Residual nitrogen time (RNT) is a time penalty added to a repetitive dive to account for the nitrogen you still carry from the previous dive. Your actual bottom time plus RNT is your total nitrogen time for that dive.

Do I still need tables if I dive a computer?

Computers track this continuously and most divers rely on them, but theory exams still test the table logic because it teaches how nitrogen loading and off-gassing actually work.

Dive theory, decoded

How to Read Dive Tables

Dive tables look like a wall of numbers, so people memorize steps instead of understanding what the table is actually tracking: nitrogen in and nitrogen out. Get that, and the table becomes obvious.

What a table is really doing

Every table is a model of theoretical nitrogen loading. It keeps you inside your no-stop limits and helps you plan a second or third dive without loading too much nitrogen. That is the entire job. Four terms carry it:

Term	What it means
No-stop limit (NDL)	Max time at a depth before a decompression stop becomes required. Deeper = shorter.
Pressure group	A letter/level standing in for how much nitrogen is still in your tissues.
Surface interval	Time on the surface off-gassing. It moves you to a lower group.
Residual nitrogen time (RNT)	A time penalty carried into the next dive for leftover nitrogen.

The three-step logic for repetitive dives

Almost every table problem is the same three moves:

First dive: depth + time gives you an ending pressure group (how much nitrogen you finished with).
Surface interval: time out of the water drops you to a lower group as you off-gas.
Repetitive dive: your new group gives a residual nitrogen time, which you add to your planned bottom time. That total, not just your bottom time, is what counts against the no-stop limit.

Where people lose marks: forgetting step 3. On a repetitive dive it is actual bottom time plus residual nitrogen time that must stay under the limit, not bottom time alone. Miss the RNT and every repetitive-dive answer comes out wrong.

Tables vs computers

A dive computer runs this same model continuously and in real time, which is why most divers use one. But the exam still tests the table method, because it forces you to understand nitrogen loading and off-gassing instead of just reading a screen. Learn the logic and the computer makes more sense too.

One note: the exact numbers (no-stop limits, group letters) differ between tables and agencies, so do not memorize a specific chart. Learn the method above and you can read any table you are handed. The related gas math, like maximum operating depth, works the same way: understand the system, not the numbers.

Drill it until it is automatic

Try the free practice quiz, or get the full guide with worked table logic and a 60-question exam.

Free practice quiz Get the guide ($29)

Frequently asked questions

Why does depth cut the no-stop limit so fast?

Nitrogen loads faster the deeper you go, because the partial pressure of nitrogen rises with depth (Henry's law). Double the loading rate and you hit the limit in far less time.

What does a safety stop have to do with the table?

A safety stop (commonly a few minutes around 5 metres) is added conservatism to off-gas a little before surfacing. It is standard practice even when the table says you are within limits.

Is the table logic the same for nitrox?

The method is identical; nitrox just uses its equivalent air depth (less nitrogen) to look up limits, which is why it earns longer no-stop times. See the nitrox guide.

Independent study aid for exam preparation only. Not affiliated with or endorsed by any certification agency, and not a substitute for certified training. Exact table values vary by agency; always use your own tables or computer and dive within your training.